Identification by HPLC of epiT produced from HEK-293 cells stably expressing 17α-HSD. (A) Elution profile of non labeled steroids; 4-dione (1st peak), testosterone (2nd peak) and epitestosterone (3rd peak). (B) Products extracted from enzymatic assay done with cells stably expressing 17α-HSD, substrate is 4-dione. (C) Products extracted from enzymatic assay done with purified enzyme; substrate is 4-dione. Separation and identification of metabolites were performed as described in Materials and Methods.